Recovery of wastes from glass grinding and polishing operations



Patented Sept. 29, 1942 UNITED STATES PATENT OFFICE- aza'tsza; i we,"renam ear No Drawing. Original application April 13, 1937,

Serial No. 136,699, now Patent No. 2,182,384,

dated December 5, 1939. Divided and this application December 4, 1939,Serial No. 307,413

8 Claims. (Cl. 23-119) The present invention relates to the preparationof alkali silicates and it'has particular relation to the preparation'oisodium silicate.

One object of the invention is to provide a simple and economicalprocess of forming sodium silicate and simultaneously to provide faceswhich formany purposes are objectionable. It is customary to subjectsuch plates to a grinding or surfacing operation in order to remove theobjectionable irregularities and to provide substantially smooth, planesurfaces.

' This operation is conducted by cementing the plates with plaster ofParis to moving tables or cars which are passed successively under aseries of revolving cast iron blocks or runners the grinding operation,but instead is replaced by cheaper and relatively impure river sandswhich initially contain many impurities, such as iron compounds and thelike often in intimate association with the silica.

The relatively coarse sand initially employed in roughing off thesurface of the glass gradually becomes brokenand worn down during thecourse of operations. At the same time, becomes charged with fine flakesor chips of glass and iron which respectively are abraded from thesurface of the glass and the iron grinding discs or runners. Duringoperations, the sand is subjected from time to time to a classifyingoperation in any convenient form of apparatus in order to separaterelatively fine particles from the coarser particles. The coarsematerial is then returned for use in making the coarser cuts upon theglass surface. The fine material is advanced to a later stage and usedfor smoothing up the partially ground surfaces. Ultimately the sand isground to such fine state of subdivision and is so contaminated withiron and other impurities originally present in the sand,

posal is highly objectionable.

chips from the glass, gypsum from the polishing tables, oil from themachinery, dust from the factory, and other materials that it can nolonger be used for grinding operations.

Typically, the discarded sand is of the following composition:

silica of high purity and in a very fine state of Percent sub-division.vSand grains 70-85 This and other objects will be apparent fromv Glassdebris 15-25 consideration of the following specification and 10 Gyps m0.3-7

. the accompanying claims. Rouge 02 0.5

In the manufacture of plate glass, the plates Metallic iron 1-2.5

as initially obtained have relatively rough sur- Organic matter TraceThis impure material has heretofore been finally discarded as uselesswaste, either by dumping it into a stream or by pilling it upon anyavailable land adjacent the factory. Either mode of dis- In case thematerial is allowed to run to waste in a stream or other body of water,contamination, of course, results. This in many cases is prohibitedbylaw. The volume of sand employed in grinding plate glass is very greatand it usually exceeds the amount of sand employed in the compounding ofthe glass. Accordingly, it is evident that an immense volume of waste isproduced and if it is simply piled up, in course of time, much space isrequired for its storage.

The present invention is based upon the discovery that sand from glassgrinding operations may by suitable treatment be purified of most of theobjectionable impurities to produce a finely ground silica of greatvalue for many purposes.

If sand of high purity, e. g. melting sand, is used for grindingoperations, iron, the major portion of which is metallic iron abradedfrom the runners, is the most objectionable material and is usuallypresent in amounts of about 1 to 2.5% based upon solids content. Oneconvenient method of removing the iron involves subjecting' the wastesfrom the polishing operations to the action of a magnetic separator. Bythis method, most of the free iron resulting from abrasion of thegrinding discs is removed. In the event magneticseparation is employed,the operation should succeed the conclusion of the grinding operationsas promptly as possible in order to obviate excessive oxidation of thefinely ground iron. The residual non-magnetic oxides or other compoundsof iron may be removed by digestion of the slurry or sludge of finelydivided sand with an acid, such as hydrochloric acid,

iron abraded from the runners, rouge, fine glass sulphuric acid,tartaric acid, nitric acid, or the like following by washing to removethe resultant soluble iron compounds.

A third method of removing particles of iron and iron oxide involvesfroth flotation. In this operation, a selective promoter of flotationwhich will increase the fiotability of the particles of iron isemployed. If desired, the iron may be given preliminary treatment inorder to increase its flotability. Such treatment includes conventionalsulfldation. It also includes treatment of the slurry of impure sandwith copper sulphate presumably in order to deposit thin films of copperupon the surfaces of the particles of iron. The copper coated particlesmay then' be floated without difficulty in the same manner as copper.The copper coated particles may also be given sulfide treatment with asoluble sulfide, such as sodium sulfide, hydrogen sulflde or the like.The flotation is then effected by methods similar to those employed inthe concentration of low grade ores of metal, for example, a frothingagent, such as pine oil or the like, together with a flotation promoter,such as xanthate, mercaptobenzothiazol, or the like are added and' theslurry is then blown with air.

to form a froth which carries most of the iron.

The removal of the iron by chlorination is also contemplated as beingwithin the purview of the invention. In such method, the waste sand ispreliminarily freedof water by conventional methods and then exposed tothe action of chlorine gas at a fairly high temperature; e. g., 1400 to1800 deg. F. Chlorination of the iron to form iron chloride results andthe latter at the temperature of operation is volatilized and leaves thesilica and other relatively inert materials in substantially iron-freestate. Two or more of these methods may be combined. For example, themajor portion of -the metallic iron may be removed magnetically, or byfroth flotationand any residue of iron or iron. compounds then removedby digestion with acid.

The operations involved in the acid purification of a waste grindingsand may be summarized as follows:

. 1. Collect the slurry from grinding operations in large tank or othersuitable container.

2. Allow the slurry to settle and decant the clear liquid. The watercontent is thus reduced from 90 per cent to 50 per cent.

3. Agitate the thickened slurry in the same tank or in a smaller tank asmay be convenient with an acid (i. e. hydrochloric or sulfuric). Enoughacid is usually added to give an acid concentration of about 5 per cent.

4. Wash sand preferably by decantation.

possible to obtain a product consisting of practically pure silica in avery. fine state of subdivision and of great value for commercialapplication. In suchprocess, the metallic iron may if desired be removedas a preliminary step. This has been done with an electro-magnet, or bydis- .solving it in acid, hydrochloric or nitric acids,

for example, being satisfactory. The iron may also be removed byflotation."

After removal of the major part of the metallic iron, it is preferableto reduce the amount of water in which the waste is suspended (fromabout 90 per cent) to about 50 per cent. This is not necessary, but issaving of chemicals in subsequent treatment.

The second major step consists in treating the waste with caustic alkalior alkali carbonate. Caustic soda or-sodium carbonate are satisfactory.This operation is conducted in a suitable vessel and may be effected atnormal room temperatures, or the temperature may be elevated, dependingupon conditions. At lower temperatures the reactions proceedmore slowlybut the advantage of more rapid action at higher temperatures is offsetby the greater cost. Naturally, in cold weather practical-expediencywill require heating. It is advisable also to provide 5. Separate sandfrom water; e. g. by filtration.

6. Dry finely divided product.

If removal of all or a part of gypsum and the glass constituents, suchas calcium, silica and sodium is desired, the process should alsoinclude treatment of the waste sand with an alkali, such as sodiumcarbonate or sodium hydroxide;

sodium silicate may, also, be employed. In such' the concentration ofthe alkali is maintained.

relatively low, e. g. a out 5% of sodium hydroxide or 10% of sodiumcarbonate, the ground silica is not substantially attacked. It thusbecomes means of stirring this reaction mixture, to insure contactbetween the alkali and each solid particle of the mass.

At temperatures attainable in open vessels concentrations of 5 per centof caustic soda or 10 per cent of sodium carbonate are satisfactory.Higher concentrations may be used (e. g. 10 per cent caustic soda or 15per cent sodium carbonate). However, as the concentration increasesthere is an increasing tendency to dissolve silica as well as glass.Substantial reduction of the concentration results in material slowingup of the reaction. At higher temperatures, such as may be attained byconducting the reaction'ln an autoclave, lower concentrations may beemployed.

A variation of the alkali treatment involves simple digestion of thewaste sand with water in an autoclave at a fairly high temperature, e.g., that corresponding to steam under or 200 pounds pressure per squareinch. As a result, a part of the alkali from the glass is hydrolized andthen tends further to break down the latter. A small amount ofadditional alkali may also be incorporated if desired.

Several chemical reactions occur in this alkaline treatment. Forexample, if sodium carbonate is the alkali used, the gypsum (CaSO-aZHzO)is decomposed to form calcium carbonate (CaCOa) and sodium sulphate(NazSOr). (The sodium sulphate is soluble in water and is removed by thesubsequent washing and the cal- .cium carbonate is later dissolved inacid and likewise removed.)

The alkali also reacts chemically with the fine particles of glass inthe waste. The chemical reactions involved appear to be complex but themajor soluble product seems to be sodium silicate and it is evident thatthis is formed chiefly from the silica of the glass.

surfaces with sand as an abrasive, which method comprises treating a themixture with aqueous caustic soda from about to percent concentrationfor a period of time sufllcient preferentially to react with the glasscontent to form a water solution of sodium silicate while leaving mostof the sand unattached and then'removing the solution from the insolubleresidues.

2. A method of removing finely ground glass from a mixture ofpulverulent glass and sand resulting from the surfacing of glass plateswith a slurry of sand as an abrasive, which method comprises treatingthe mixture with carbonate of soda of about 5 to percent concentrationfor a period of time suflicient preferentially to react the glasscontent to form a water solution of sodium silicate, while leaving mostof the sand unattacked, then removing the resultant solution of sodiumsilicate from the insoluble residue.

3. A process of removing finely ground glass from the mixture ofpulverulent glass and-silica resulting from the surfacing 01' glassplates with sand as an abrasive, which process comprises digesting themixture with hot water under pressure in an autoclave to form sodiumsilicate solution from the glass, then removing the solution from theresidue of silica and insoluble glass constituents.

4. A process of forming sodium silicate comprising digesting with adilute water solution of alkali, the mixture of finely-divided sand andglass resulting from the surfacing of plate glass with a water slurry ofsand as an abrasive for a time sufllcient to efiect selective reactionof the alkali with the glass content of the mixture to form alkalisilicate while-leaving most of the sand unattacked.

5. A process of forming sodium silicate, which process comprises.digesting with a water solution of soda ash of not greater than about 15percent concentration the finely-divided and thoroughly admixed sand andglass resulting from the grinding 0! plate glass with a slurry of sandas an abrasive, tor. a' period of time suilicient selectively to reactwith the glass content of the mixture to form a water solution oi.sodium silicate while leaving the sand content substantially unreacted.

-6. A process of forming sodium silicate, which process comprisestreating a finely-divided and very intimate mixture of sand and glassresulting from the grinding of glass plates with a. slurry of sand of 7glass batch purity as an abrasive, in order to remove finely-dividediron resulting from the grinding operation, then digesting the mixturewith a dilute water solution of alkali to obtain selective reaction ofthe glass content to form a solution 0! sodium silicate, while leavingthe finely divided sand substantially unattacked,then removing thesolution from the unreacted residue.

7. A process of forming sodium silicate, whichprocess comprisesdigesting the finely-divided and thoroughly distributed mixture of sandand glass resulting from the grinding of plate glass with a slurry ofsand of glass batch purity as an abrasive with relatively dilute aqueoussodium hydroxide for a period of time sufllcient to obtain selectivereaction oi. the glass content without substantial attack upon the sandcontent.

8. A process comprising digesting a mixture of sand and glass, resultingfrom the abrasion 01 glass plates with sand as a water slurry until thesand is broken down to a pulverulent state, with a medium from the groupconsisting of water at superatmospheric pressure and elevatedtemperature and alkaline solutions, for a period of time suillcient topreferentially react with the glass content of the mixture to formalkali silicates from the glass while leaving the sand unattacked, andthen removing the resultant solution of alkali silicate from thesiliceous residue.

- ROB nor McGREGQR.

